There is a general need of extremely hard materials for many fields of application. These extremely hard materials are also called “superhard” when they exhibit a hardness of >40 GPa. These materials are used in a variety of applications such as tools for cutting, turning, milling, drilling, sawing, grinding operations, and the like. The hard materials may also be used for their wear, abrasion and erosion resistance when working as bearings, seals, nozzles or in similar cases. The materials may be working on, or being in contact with, many materials such as cast iron, steel, non-iron metals, wood, paper, polymers, concrete, stone, marble, soil, cemented carbide and grinding wheels of aluminum oxide, silicon carbide, diamond, cubic boron nitride, and the like. As being the hardest material known, mono- or polycrystalline diamond is suitable for these purposes. Other common materials used for their hardness are for instance cubic boron nitride (CBN), boron carbide and other ceramics and cemented carbides, however only diamond or CBN containing materials can reach the superhard group of materials.
It is well known that carbon in the diamond structural form is thermodynamically unstable at ambient temperatures and pressures. Nevertheless the decomposition of diamond to graphite (graphitization) is hindered by kinetic reasons and diamonds found in nature have existed for millions of years. However, by increasing the temperature, graphitization of diamond crystals will occur with a process starting from the surface, where the energy to overcome the kinetic hindrance is highest and where defects or catalytic effects from other surface impurities or the atmosphere will influence this process.
By heating in air it is well known that the decomposition and oxidation of diamonds will take place at temperatures as low as 600–700° C. Carbon solving metals like cobalt may catalyze a reaction already at about 500° C. The graphitization process is delayed to higher temperatures in vacuum or inert atmosphere and diamonds are most stable in hydrogen gas atmosphere, where the environment is strongly reducing. High quality diamond is stable for long times to about 2000° C.
Different composite bodies with bonded diamond particles are known. The diamond particles may be bonded by a matrix comprising metal and/or ceramic phases and produced by sintering diamond particles in a matrix of such materials, or bonded by the infiltration of silicon or silicon alloys into the diamond body, for instance.
By heating a body of diamond powder in a furnace to high temperatures during extended times, a small amount of uncontrolled and undesirable graphitization might occur depending also on the pressure. In previously reported processes to form densely sintered diamond composite bodies this has been an unwanted effect and different ways of avoiding this have been used. The most practiced technique is to use high pressures during the sintering step and stay in the diamond stable area of the phase diagram at 1300–1600° C., in high-pressure chambers with pressures of 30,000–60,000 atm (HP/HT). See for instance FIG. 4, in U.S. Pat. No. 4,151,686; for a diamond-graphite phase diagram.
The required extremely high pressures are only achieved by specially made presses and dies. The consequences are high production costs, limited production capacity and limited shapes and sizes of the diamond composite bodies.
There are also methods for production of diamond bodies using lower pressures than needed for the diamond stable area, from about a minimum of 500 psi (about 34 bars) and above, e.g. the method according to U.S. Pat. No. 4,124,401.
In the case where the pressure has been in the graphite stable region, for instance using a furnace with protective inert atmosphere, graphitization has been minimized by using short times at high temperature or reducing the sintering temperature for solidification of the body. An example of the latter is to use metal alloys of silicon that have a significantly lower melting temperature than that of pure silicon.
Several patents reveal techniques to produce materials containing diamond, silicon carbide and silicon without using high pressures. There are a number of variations of the process, mainly concerning the use of different carbonaceous materials (hereafter referring to all kinds of non-diamond carbon materials like carbon black, carbon fibres, coke, graphite, pyrolytic carbon etc). In principal the following steps are followed.
A. Non-coated diamond particles or normally, carbon-coated diamond particles and carbonaceous materials are used as precursor materials. According to the examples, U.S. Pat. No. 4,220,455 starts with adding a thin layer (500–1000 Angstrom) of carbon on the diamonds by a pyrolytic reaction. The pyrolysis is done in vacuum for a few minutes by feeding natural gas or methane, into a furnace with diamond particles at 1200° C. Sometimes diamonds without a pyrolytic carbon layer are used, as in U.S. Pat. No. 4,381,271, EPO 0 043 541, EPO 0 056 596 and JP 6-199571A. Both carbon-coated and non-coated diamonds are mixed with carbonaceous materials as a main source of carbon e.g. carbon black, short carbon fibres or cloth and a binder etc. before the forming of green bodies.
B. Forming of green bodies of the diamond particle/carbon material mixture is done in a mould. The green bodies contain additionally solvents and temporary or permanent binders to facilitate the forming and to increase the strength of the green body.
C. Work-pieces are made by heat treating the green bodies. Some binders are vaporised without leaving any residues e.g. paraffin, other binders are hardened leaving a carbonaceous residue in the work-piece, e.g. phenol-formaldehyde and epoxy resins.
D. Infiltration of the porous work-piece with molten silicon is done to form silicon carbide in a reaction between the carbon and the silicon. The heat treatment is done in such a manner as to minimise the graphitization of diamond, which is considered harmful. In the examples of U.S. Pat. No. 4,220,455 silicon is infiltrated in vacuum when the body is in a mould, at a temperature between 1400°–1550° C. for 15 minutes, during which time the reaction between silicon and carbon is completed. U.S. Pat. No. 4,242,106 uses a vacuum of 0.01–2.0 torr during the infiltration. The required time, depending largely on the size of the body, is determined empirically and takes about 15–20 minutes at a temperature above 1400° C., or 10 minutes at 1500° C. U.S. Pat. No. 4,381,271 uses carbon fibres to promote the infiltration of fluid silicon by a capillary action. In most of the patents infiltration is made in a mould. In some earlier patents the infiltration is made outside the mould, like in EPO patent 0 043 541.
Not only silicon has been used for the infiltration and bonding of diamond particles. Several patents describe using silicon alloys instead of pure silicon. U.S. Pat. No. 4,124,401 describes a hot-press method using an eutectiferous silicon alloy for infiltration. U.S. Pat. No. 5,266,236 uses a boron-silicon alloy in a HP/HT method. U.S. Pat. No. 4,664,705 discloses a method that infiltrates a silicon alloy through a PCD body, where the binder has earlier preferably been leached out.
The processes where carbon-coated or non-coated diamonds are mixed with carbonaceous materials might have disadvantages, e.g. difficulties in preparing homogeneous mixtures of these materials, difficulties of silicon infiltration due to very small pore sizes and necessity of special equipment for preparing homogenous mixtures.
In the patent RU 2064399 the addition of carbon by pyrolysis is done only after the forming and production of the work-piece. A preformed work-piece of diamond particles or a mixture of diamond particles and carbide grains as filler, is produced with a temporary binder. The binder is evaporated and the work-piece is placed in a reactor, where pyrolytic carbon is deposited on all grains of the body by a pyrolytic reaction from a gas phase, e.g. methane at 950° C. for 10 h. After this follows silicon infiltration. The drawbacks of this process are the use of a great amount of hydrocarbon gas and that the processing time is rather long. If carbide grains are used as fillers, the same problems of homogenisation as mentioned above appear.
There are some methods for improving the diamond composite materials produced by the earlier described techniques. One of them is to arrange the diamond particles as graded structures of concentration and size in the material, thereby affecting some properties and also the field of application. A method of making a size graded material by sintering at high pressure and high temperature is disclosed in the patent EPO 0 196 777. The grain size and/or packing density are varied in layers between the front face and rear face to get different wear resistance in these parts. The drawback of this method is that since it uses high pressure-high temperature, the production of the material is more expensive and requires special equipment and there are size limitations.
There are also a number of patents using different amount of diamond in different parts of the composite body. The following patents U.S. Pat. No. 4,242,106; U.S. Pat. No. 4,247,304; U.S. Pat. No. 4,453,951; EPO 0 043 541; EPO 0 056 596 describe the production of layered structures of a final material with a diamond composite layer in contact with a supporting silicon carbide or silicon carbide-silicon substrate, for instance. U.S. Pat. No. 4,698,070 describes the production of a composite with a diamond containing portion and a core portion united by a matrix of silicon carbide and silicon. Additional particle layers with other diamond concentration may also be provided and placed e.g. in corners, on the top, in the core.
Generally the drawback of layered materials with different diamond size or concentration is that there may be differences in physical/mechanical properties in the diamond containing and supporting layers, e.g. thermal expansion coefficient and E-modulus, might cause unwanted stress situations at the interface and thereby weaken the composite under stress. Diamonds have a relatively low tensile strength and low toughness, and a distinct difference in diamond content in different parts joined by an interlayer may also affect the fracture resistance of composites. None of the methods described earlier result in bodies with prior specified distribution of diamond particles of different size throughout the material volume, with uniformly changing properties.
The composites of U.S. Pat. No. 4,220,455 consist of a mixture of diamond particles of different size in the whole body, i.e. the composite does not have layered structures. The particular sizes used are chosen depending on the desired packing of particles and resulting body. For most abrasive applications particles no greater than about 60 μm are preferred. Preferably to maximise the packing of the particles they should contain a range of sizes, i.e. small, medium and large.
None of the methods described above use graphitization intentionally. Instead the graphitization is considered as harmful and unwanted.
In RU patent 2036779 a preform is moulded of diamond powder eventually together with water or ethyl alcohol, placed in a furnace and impregnated with liquid silicon at 1420°–1700° C. in argon or vacuum. In the process the surface of the diamond grains is minimally graphitized, so the greater part of the diamond is still unchanged. This minor amount of graphite, reacts in contact with infiltrated silicon creating a thin surface layer of silicon carbide that keeps back any further formation of diamond to graphite during the used process. The drawback of this process is poor control and there is no method for governing the amount of produced SiC, residual silicon or porosity left in the composite.
Thus in these previous patents there is no teaching about a well-controlled step of adding carbonaceous materials to a diamond body and intentional graphitization step for production of materials with desired amount of diamond, silicon carbide and silicon, with low porosity and no graphite.